Egg. 



121 



that do not become ova. Secondly, the structure of the 

 mature ovum agrees iii every point with that of a single cell : 

 it is made up of a mass of protoplasm in which is imbedded 

 a nucleus ; the reticular structure of the protoplasm and of the 

 nucleus is exactly paralleled in other tissue cells ; and, finally, 

 the cells of many tissues (all, probably, according to some 

 histologists) possess a centrosome — that body which plays so 

 important a part in cell division. Thirdly, the way in which 

 an ovum divides is like that of other cells; the complicated 



Fig. 6o. — Amceboid movements of young egg-cells. 



CI to cs, egg of a Cat, in five successive stages (after Pfliiger) ; D, ditto of trout ; e, of 

 a Hen; F, of Man. 



process of Karyokinesis is copied, so to speak, in the ovum 

 (cf. Figs. 64, 65), detail for detail, from the method of other 

 cells. It is true that in many cells a simpler mode of cell 

 division, not found in ova, is often met with, i.e. direct or 

 amitotic division, in which the nucleus simply constricts and 

 divides, without the complicated figures of Karyokinesis (or 

 Mitosis, as it is sometimes termed) ; but the indirect method is 

 the more characteristic mode of division of cells in general. 



Another highly important generalisation to be borne in 

 mind in consideiring the single-celled character of the ovum 

 is its consequent likeness to a single-celled organism, such as 

 an Amoeba. The body of the most advanced animal is 



